1. Field of Invention
This invention relates to automatic control of heating, ventilating, and air conditioning (HVAC) systems and more particularly to automatic control of HVAC systems to reduce both the energy required for system control and the actual energy consumption required for maintaining desired environmental temperature conditions.
2. Description of Prior Art
As may be known, HVAC systems are used for controlling the environmental temperature conditions in one or a plurality of structurally enclosed living spaces, i.e. those spaces inhabited by humans. In general such HVAC systems include separate sources of heating and cooling which may be individually ducted to the temperature controlled space. The ducting systems are varied and include among others, systems having individual hot and cold ducts communicating directly with each of the controlled spaces through space dampers which are thermostatically controlled at the space site to modulate the amount of hot and cold air discharged into the space. Such systems generally include a source of conditioned return air from the spaces in addition to outside fresh air, the return air and outside air being modulated to provide a mixed air supply which is forced through the heating and cooling sources within the respective hot and cold ducts. An HVAC system of this type is shown and described in a U.S. Pat. No. 4,013,118 to Zimmer et al, wherein the amount of heating and cooling provided in the hot and cold ducts are determined by the highest heat and cold demand space (referred to by Zimmer as zones) among the plurality of spaces being controlled. The mixed air temperature is controlled by dampers located within respective outside air and return air ducts which are inversely positioned in response to the deviation of a sensed mixed air temperature from a selected set point. In operation this system receives signals representative of the space temperature error in the worst case hot and cold spaces and compares the signal magnitudes with fixed duct reference temperatures to provide corresponding heat and cool demand signals which turn on sequential heating and cooling stages.
A similar multi-zone temperature control system is shown and described in U.S. Pat. No. 3,901,310 to C. S. Strawn. As with the patent to Zimmer et al the system of Strawn controls hot and cold deck temperatures in response to the hottest and coldest of the plurality of zones, and controls the various zones to a selected reference temperature within each zone provided by the setting of a zone thermostat.
A more recent HVAC control for a multi-zone, or multi-space system is disclosed in U.S. Pat. No. 3,949,807 to H. J. Tyler in which the temperature within each of a plurality of individual spaces is maintained with hot and cold ducted air supplied from heating and cooling sources, each controlled by a master controller as in the systems disclosed by Zimmer et al and Strawn. The control system of Tyler, however, provides each of the controlled spaces with heat and cool set point temperatures separated by a temperature deadband. Mechanical heating or cooling is provided only in response to a proportional error representative of the deviation of the worst case temperature space above or below the corresponding cool or heat set point, and the degree of heating or cooling is controlled by a fixed proportional gain control loop. For the condition in which all space temperatures are within the temperature deadband, the control system modulates the outside air and return air dampers through a straight proportional gain, mixed air temperature control loop to obtain the required mixed air temperature within the hot and cold decks necessary to maintain the plurality of space temperatures to a fixed space temperature set point at one end of the temperature deadband closest to the heat set point temperature, so long as the outside air enthalpy is within prescribed limits. The mixed damper position signal is similarly proportional to the deviation of the space from the mean mixed air temperature air reference, and the mixed air dampers are modulated from a minimum open position to a full open position in response to less than one degree of space temperature error. The control system of Tyler provides a measure of energy conservation in that the hottest and coolest spaces cannot control activation of the mechanical heating air cooling until such spaces have exceeded the temperature deadband set points, however, once activated the system provides proportional control of the mechanical heating and cooling in the same way as the prior systems.
As may be known, a proportional control system requires an ever present temperature error signal, i.e. "droop error" to provide the necessary output error signal to control the associated heating or cooling sources (steam or chill water generators). The magnitude of the error signal is equal to the difference temperature between the actual and reference duct temperatures divided by the proportional gain constant, and the signal magnitude limit cycles about a mean value between the actual and duct reference values, such that the magnitude of the "droop error" is dependent on the magnitude of the proportional gain. Small values of proportional gain result in a large "droop error" and the larger the magnitude of the proportional gain the more severe the limit cycle amplitude of the error signal due to the inherent time lags and thermal response of the system. Typical peak to peak limit cycle temperature amplitudes of 20.degree. are common, such that the duct temperature may limit cycle between 55.degree. and 75.degree. in an attempt to maintain a mean duct temperature of 65.degree.. In addition to the obvious waste of control energy in cycling the heating or cooling source through these temperature extremes, the actual energy consumption of the heating and cooling sources required to maintain space temperature is similarly increased.